We propose a method to simulate a-Si and a-Si:H using an ab initio approach based on the Harris functional and thermally-amorphisized periodically-continued cells with at least 64 atoms. Hydrogen incorporation was achieved via diffusive addition. In preparing samples that may simulate the distributions of atoms in the amorphous materials, simulated annealing calculations were carried out from given starting conditions using short and long time steps. The different time-steps led to samples having distinctly different topological disorder. The radial distribution functions (partial and total) of the resulting samples were calculated and compared with measured distributions; the agreement is very good. These comparisons allowed some tentative conclusions to be made as regards the kind of disorder prevailing in the real samples. In addition, we studied the effect of the topological disorder on the electronic densities of states of the samples; the passivating effect of hydrogen can be observed.